The invention relates to a high pressure metal halide lamp comprising:
a light transmitting discharge vessel, enclosing a discharge space, sealed in a gas-tight manner, in which tungsten electrodes are disposed, which are connected to current conductors which extend to the exterior; PA1 a filling in the discharge vessel comprising a rare gas, a buffer gas and at least one halide chosen from the halides of hafnium and zirconium.
Such a high pressure metal halide lamp is known from EP-0 492 205-A2, which corresponds to U.S. Pat. No. 5,323,085.
The known lamp contains a mixture of halides of one of the metals hafnium and zirconium, i.e. a mixture of the iodide and the bromide, particularly in a mol ratio of 0.2 to 5.
Although the known lamp was destined to yield light having a colour temperature of between 4000 and 9000 K., the lowest colour temperature described is 5200 K. and the highest 6200 K. The lamp is furthermore destined to have, and has indeed, a high colour rendering index Ra and a good R.sub.9 index value, indicating a good strong-red rendering.
The known lamp has a relatively low luminous efficacy of about 70 lm/W at a relatively high power consumption of 400 W, although it is generally known that the luminous efficacy of a discharge lamp is generally high at relatively high power consumption.
The life of the known lamp is relatively short, a few hundreds of hours.
The known lamp comprises cesium. Cesium is known to lower the reignition voltage of discharge lamps, without having a substantial influence on the light generated. The lamp may furthermore comprise additives like rare earth metals, cobalt and/or nickel in order to improve the quality of the light generated. These additives are shown, however, to have a slight influence, only. Other additives investigated are said to have no favourable effect.
In the non-prepublished European patent application 92 20 36 50.4 (which corresponds to U.S. Pat. No. 5,382,873) high pressure discharge lamps are described with or without internal electrodes. The lamps comprise an halide of hafnium and/or zirconium as the light generating species. During operation of the lamps the halide is evaporated and decomposed in a high temperature region of the discharge. A supersaturated metal vapour is then formed from which metal particles originate by condensation. These particles generate light by incandescence.
The electroded lamps of this non-prepublished application have a long life as compared to electroded discharge lamps having a volatile tungsten compound as the light generating species which generates incandescent tungsten clusters after having been decomposed: a few hours as compared to a few minutes.